The electronics industry desires a metallized coating on dielectric materials for functional and aesthetic purposes. A particularly important technological area where the techniques of metallization of dielectric materials have found applicability is in the manufacture of printed circuit boards, where metallization is used to provide patterned, conductive circuitry on substrates with dielectric materials. Metallization of dielectric materials may come into play at a number of steps in the overall process of printed circuit board manufacture. One area of substantial importance is the electroless metallization of through-holes.
Typically, printed circuit boards are planar and have printed circuits on both sides. The boards may be multi-layer and contain laminates of dielectric substrates and conductive metal such as copper, where one or more parallel inner layers of conductive metal are separated by dielectric substrates. Exposed outer sides of the laminate contain printed circuit patterns as in double-sided boards and the inner conductive layers may themselves comprise circuit patterns. In double-sided and multi-layer printed circuit boards, it is necessary to provide conductive interconnection between or among the various layers or sides of the boards. This is achieved by providing metallized, conductive through-holes in the boards communicating with the sides and layers requiring electrical interconnection. Typically the method for providing conductive through-holes is by electroless deposition of metal on the dielectric surfaces of the through-holes drilled or punched through the boards.
Electroless deposition of metal on dielectric surfaces typically involves applying a material which is catalytic to the electroless plating process to the dielectric surfaces. This is known as “activation” of the through-hole surfaces. Such catalytic material may be a noble metal such as palladium. When plating through-holes with copper, the catalyst is often a colloidal solution of palladium and tin compounds. The tin functions as a protective colloid for the catalytic palladium. In many cases the activation is followed by an “acceleration” step which serves in some manner to expose or increase exposure of the active catalytic species.
Notwithstanding the fact that the topography of the through-hole surfaces can be such, e.g., roughened or pitted, as to promote adhesion of catalysts for electroless metal deposition, the properties of the dielectric substrate material may still lead to poor adhesion. A primary example of this is found in the glass-filled epoxy resins which are used extensively in the printed circuit board industry as the dielectric substrate. Poor palladium catalyst adsorption leads to incomplete or too thin coverage of subsequently electrolessly plated copper in the through-holes. A possible explanation for this is that the glass fibers have a highly negative surface charge and do not attract the typical tin-palladium catalyst which also carries a negative charge. However, the problem of poor metal coverage in through-holes is not restricted to glass-containing dielectric substrates and also occurs in substrates with any number of a variety of non-glass-containing, dielectric materials used as circuit board substrates. Complete metal coverage of through-holes is important.
In response to the problem of poor metal coverage of epoxy-glass substrates, the printed circuit board industry addressed the problem by using a process known as “conditioning” prior to application of the catalyst. Conditioning agents are compounds or mixtures of compounds which function to improve the adsorption of activating material on substrate surfaces to improve subsequent electroless metal plating quality. The exposed through-hole surfaces are coated with the conditioning agents and the catalytic species builds up on the coating and adheres to the surface.
Although conditioning through-hole walls results in improved metal coverage in contrast to non-conditioned through-holes, coverage of through-holes in glass-epoxy substrates as well as other types of dielectric substrates with many current conditioning agents does not always meet industry standards. Accordingly, there is still a need for an improved conditioner for electroless metallization of dielectric substrates.